The present invention relates to a method for manufacturing a crystal with at least two segments, wherein adjacent segments differ by at least one characteristic. The different segments can, for example, be of different materials, or have different doping (different concentration or different doping agent).
In particular in laser technology, optical components that are composed of different crystal segments offer considerable advantages in use. Different designs of compound segmented laser crystals are already known in laser diode pumped and lamp pumped solid state lasers. Thus, for example, multiple wavelength lasers (for example Nd:YAG-Er:YAG for wavelengths of xcex=1064 nm and 2940 nm) can be produced with segmented laser crystals. The known segmented laser rods have different doping concentrations over the length of the rod.
The segmented crystals on the market are manufactured with the aid of two different methods.
Thus, for example, the Verneuil crystal growing process is used for manufacturing ruby laser rods with undoped ends. In this case, the flame of an oxy-hydrogen burner is directed perpendicularly downwards onto a rotating seed crystal. A flow of oxygen xe2x80x9centrainsxe2x80x9d the pulverized starting material. This is melted in the flame and results in crystal growth at the upper end of the crystal.
The advantage of the Verneuil process, which is also called the flame-melting process, is the lack of a crucible. The large temperature differences, which generally result in stresses and non-homogeneity in the crystal, are, however, disadvantageous. In addition, a curved joint line between different segments is always obtained with this process.
The other process known in the technology for manufacturing segmented laser rods uses the xe2x80x9cbondingxe2x80x9d technique. Here, crystals are assembled by thermal means, by ion diffusion. The crystals segments to be joined are separately grown for this purpose, and have to be polished to a planarity of at least {fraction (1/10)} of the laser wavelength. The polished surfaces must subsequently be assembled in a crystalographically orientated manner. This process is very time consuming and can only be carried out under clean room conditions. As a result, it is also very expensive.
The object of the present invention is therefore to provide a method for manufacturing a crystal with at least two segments, which makes possible the manufacturing of segmented crystals with a high crystal quality, and as planar as possible joining surfaces between the individual segments, and which is very cost effective.
This object is solved in that the segmented crystal is grown directly from the molten mass.
This is possible using both the Czochralski method and the flux method.
In the Czochralski method, a crystal is drawn unsupported from a molten mass present in a crucible, with precise control of the drawing speed. This method of crystal growing is generally initiated using a thin seed crystal with the desired orientation. This method has the advantage that the finished crystal is no longer in contact with the crucible.
In order to manufacture a segmented crystal, a crystal with a specific composition is drawn from the molten mass, for example, with the additional assistance of an orientated seed crystal. The drawing process is then interrupted, that is to say the drawing speed at which the crystal is being drawn from the molten mass is substantially zero, and the composition of the molten mass is altered. The drawing process is then resumed and a further segment, with a different composition, grows on the crystal.
Particularly preferred is a method with these steps:
immersion of a crystal with a specific composition into a molten mass with a different composition,
crystallising on,
further drawing of a crystal segment with the different composition.
The procedure wherein the crystal is placed in the molten mass is known as crystallising on. Because, before being placed in, the crystal has a temperature well below the melting temperature, heat is dissipated via the crystal from the contact surfaces between the crystal and the molten mass, and a few grams of the molten mass spontaneously crystallise on the crystal.
It is thus of importance that after the crystal with a specific composition has been drawn, it is removed from the molten mass, and, in a further step, is then immersed in a molten mass with a different composition.
With the aid of this method, large, segmented single crystals can very easily be manufactured. With crystal growth according to this method, however, under normal growing conditions, a curved, conical growth front forms, caused by the thermal gradients and the expansion coefficients of the molten mass. This growth front follows the molten mass isotherms. This molten mass isotherm generally occurs because of molten mass convection going from the edge of the crucible to the centre of the crucible.
In most cases, however, segmented crystals, in particular segmented laser crystals, with substantially planar boundary faces are desired.
A particularly advantageous method for manufacturing a segmented crystal therefore provides that the crystal rotates about its own axis during crystal growth. By means of rotation of the crystal, the downward convection below the crystal is weakened, so the molten mass isotherm follows a more planar course.
A method is therefore particularly preferred in which the crystal rotates at the inversion rotation speed during the crystal rotation. The inversion rotation speed, which is dependent inter alia upon the diameter of the crystal, is the speed of rotation at which molten mass isotherm runs horizontally, so a horizontal growth front is also constructed.